Intents and Intent Filters

An Intent is a messaging object you can use to request an action
from another app component.
Although intents facilitate communication between components in several ways, there are three
fundamental use cases:

For versions earlier than Android 5.0 (API level 21), you can start a service by using
methods of the Service class. You can start a service
to perform a one-time operation
(such as downloading a file) by passing an Intent
to startService(). The Intent
describes the service to start and carries any necessary data.

If the service is designed with a client-server interface, you can bind to the service
from another component by passing an Intent to bindService(). For more information, see the Services guide.

Delivering a broadcast

A broadcast is a message that any app can receive. The system delivers various
broadcasts for system events, such as when the system boots up or the device starts charging.
You can deliver a broadcast to other apps by passing an Intent
to sendBroadcast() or
sendOrderedBroadcast().

Intent types

There are two types of intents:

Explicit intents specify which application will satisfy the intent, by supplying
either the target app's package name or a fully-qualified component class name. You'll
typically use an explicit intent to start a component in
your own app, because you know the class name of the activity or service you want to start. For
example, you might start a new activity within your app in response to a user action, or start
a service to download a file in the background.

Implicit intents do not name a specific component, but instead declare a general action
to perform, which allows a component from another app to handle it. For example, if you want to
show the user a location on a map, you can use an implicit intent to request that another capable
app show a specified location on a map.

Figure 1 shows how an intent is used when starting an activity. When the
Intent object names a specific activity component explicitly, the system
immediately starts that component.

Figure 1. How an implicit intent is
delivered through the system to start another activity: [1]Activity A creates an
Intent with an action description and passes it to startActivity(). [2] The Android System searches all
apps for an intent filter that matches the intent. When a match is found, [3] the system
starts the matching activity (Activity B) by invoking its onCreate() method and passing it the Intent.

When you use an implicit intent, the Android system finds the appropriate component to start
by comparing the contents of the intent to the intent filters declared in the manifest file of other apps on the
device. If the intent matches an intent filter, the system starts that component and delivers it
the Intent object. If multiple intent filters are compatible, the system
displays a dialog so the user can pick which app to use.

An intent filter is an expression in an app's manifest file that
specifies the type of intents that the component
would like to receive. For instance, by declaring an intent filter for an activity,
you make it possible for other apps to directly start your activity with a certain kind of intent.
Likewise, if you do not declare any intent filters for an activity, then it can be started
only with an explicit intent.

Caution: To ensure that your app is secure, always
use an explicit
intent when starting a Service and do not
declare intent filters for your services. Using an implicit intent to start a service is a
security hazard because you can't be certain what service will respond to the intent,
and the user can't see which service starts. Beginning with Android 5.0 (API level 21), the system
throws an exception if you call bindService()
with an implicit intent.

Building an intent

An Intent object carries information that the Android system uses
to determine which component to start (such as the exact component name or component
category that should receive the intent), plus information that the recipient component uses in
order to properly perform the action (such as the action to take and the data to act upon).

This is optional, but it's the critical piece of information that makes an intent
explicit, meaning that the intent should be delivered only to the app component
defined by the component name. Without a component name, the intent is implicit and the
system decides which component should receive the intent based on the other intent information
(such as the action, data, and category—described below). If you need to start a specific
component in your app, you should specify the component name.

Note: When starting a Service,
always specify the component name. Otherwise, you cannot be certain what service
will respond to the intent, and the user cannot see which service starts.

This field of the Intent is a
ComponentName object, which you can specify using a fully
qualified class name of the target component, including the package name of the app, for example,
com.example.ExampleActivity. You can set the component name with setComponent(), setClass(), setClassName(),
or with the
Intent constructor.

Action

A string that specifies the generic action to perform (such as view or pick).

In the case of a broadcast intent, this is the action that took place and is being reported.
The action largely determines how the rest of the intent is structured—particularly
the information that is contained in the data and extras.

You can specify your own actions for use by intents within your app (or for use by other
apps to invoke components in your app), but you usually specify action constants
defined by the Intent class or other framework classes. Here are some
common actions for starting an activity:

Also known as the share intent, you should use this in an intent with startActivity() when you have some data that the user can
share through another app, such as an email app or social sharing app.

See the Intent class reference for more
constants that define generic actions. Other actions are defined
elsewhere in the Android framework, such as in Settings for actions
that open specific screens in the system's Settings app.

You can specify the action for an intent with setAction() or with an Intent constructor.

If you define your own actions, be sure to include your app's package name
as a prefix, as shown in the following example:

Kotlin

const val ACTION_TIMETRAVEL = "com.example.action.TIMETRAVEL"

Java

The URI (a Uri object) that references the data to
be acted on and/or the
MIME type of that data. The type of data supplied is generally dictated by the intent's action. For
example, if the action is ACTION_EDIT, the data should contain the
URI of the document to edit.

When creating an intent,
it's often important to specify the type of data (its MIME type) in addition to its URI.
For example, an activity that's able to display images probably won't be able
to play an audio file, even though the URI formats could be similar.
Specifying the MIME type of your data helps the Android
system find the best component to receive your intent.
However, the MIME type can sometimes be inferred from the URI—particularly when the data is a
content: URI. A content: URI indicates the data is located on the device
and controlled by a
ContentProvider, which makes the data MIME type visible to the system.

Caution: If you want to set both the URI and MIME type,
don't call setData() and
setType() because they each nullify the value of the other.
Always use setDataAndType() to set both
URI and MIME type.

Category

A string containing additional information about the kind of component
that should handle the intent. Any number of category descriptions can be
placed in an intent, but most intents do not require a category.
Here are some common categories:

These properties listed above (component name, action, data, and category) represent the
defining characteristics of an intent. By reading these properties, the Android system
is able to resolve which app component it should start. However, an intent can carry
additional information that does not affect
how it is resolved to an app component. An intent can also supply the following information:

Extras

Key-value pairs that carry additional information required to accomplish
the requested action.
Just as some actions use particular kinds of data URIs, some actions also use particular extras.

You can add extra data with various putExtra() methods,
each accepting two parameters: the key name and the value.
You can also create a Bundle object with all the extra data, then insert
the Bundle in the Intent with putExtras().

For example, when creating an intent to send an email with
ACTION_SEND, you can specify the to recipient with the
EXTRA_EMAIL key, and specify the subject with the
EXTRA_SUBJECT key.

The Intent class specifies many EXTRA_* constants
for standardized data types. If you need to declare your own extra keys (for intents that
your app receives), be sure to include your app's package name
as a prefix, as shown in the following example:

Kotlin

const val EXTRA_GIGAWATTS = "com.example.EXTRA_GIGAWATTS"

Java

static final String EXTRA_GIGAWATTS = "com.example.EXTRA_GIGAWATTS";

Caution: Do not use Parcelable or
Serializable data when sending an intent that you expect
another app to receive. If an app
attempts to access data in a Bundle object but does not
have access to the parceled or serialized class, the system raises a
RuntimeException.

Flags

Flags are defined in the Intent class that function as metadata for the
intent. The flags may instruct the Android system how to launch an activity (for example, which
task
the activity should belong
to) and how to treat it after it's launched (for example, whether it belongs in the list of recent
activities).

Example explicit intent

An explicit intent is one that you use to launch a specific app component, such as
a particular activity or service in your app. To create an explicit intent, define
the component name for the Intent object—all
other intent properties are optional.

For example, if you built a service in your app, named DownloadService,
designed to download a file from the web, you can start it with the following code:

Kotlin

// Executed in an Activity, so 'this' is the Context
// The fileUrl is a string URL, such as "http://www.example.com/image.png"
val downloadIntent = Intent(this, DownloadService::class.java).apply {
data = Uri.parse(fileUrl)
}
startService(downloadIntent)

Java

// Executed in an Activity, so 'this' is the Context
// The fileUrl is a string URL, such as "http://www.example.com/image.png"
Intent downloadIntent = new Intent(this, DownloadService.class);
downloadIntent.setData(Uri.parse(fileUrl));
startService(downloadIntent);

The Intent(Context, Class)
constructor supplies the app Context and the
component a Class object. As such,
this intent explicitly starts the DownloadService class in the app.

For more information about building and starting a service, see the
Services guide.

Example implicit intent

An implicit intent specifies an action that can invoke any app on the device able
to perform the action. Using an implicit intent is useful when your app cannot perform the
action, but other apps probably can and you'd like the user to pick which app to use.

For example, if you have content that you want the user to share with other people,
create an intent
with the ACTION_SEND action
and add extras that specify the content to share. When you call
startActivity() with that intent, the user can
pick an app through which to share the content.

Caution: It's possible that a user won't have any
apps that handle the implicit intent you send to startActivity(). Or, an app may be inaccessible because of profile restrictions or settings
put into place by an administrator. If that happens, the call fails and your app crashes. To verify
that an activity will receive the intent, call resolveActivity() on your Intent object. If the result is non-null,
there is at least one app that can handle the intent and it's safe to call
startActivity(). If the result is null,
do not use the intent and, if possible, you should disable the feature that issues
the intent. The following example shows how to verify that the intent resolves
to an activity. This example doesn't use a URI, but the intent's data type
is declared to specify the content carried by the extras.

When startActivity() is called, the system
examines all of the installed apps to determine which ones can handle this kind of intent (an
intent with the ACTION_SEND action and that carries "text/plain"
data). If there's only one app that can handle it, that app opens immediately and is given the
intent. If multiple activities accept the intent, the system
displays a dialog such as the one shown in Figure 2, so the user can pick which app to use.

Forcing an app chooser

When there is more than one app that responds to your implicit intent,
the user can select which app to use and make that app the default choice for the
action. The ability to select a default is helpful when performing an action for which the user
probably wants to use the same app every time, such as when opening a web page (users
often prefer just one web browser).

However, if multiple apps can respond to the intent and the user might want to use a different
app each time, you should explicitly show a chooser dialog. The chooser dialog asks the
user to select which app to use for the action (the user cannot select a default app for
the action). For example, when your app performs "share" with the ACTION_SEND action, users may want to share using a different app depending
on their current situation, so you should always use the chooser dialog, as shown in Figure 2.

To show the chooser, create an Intent using createChooser() and pass it to startActivity(), as shown in the following example.
This example displays a dialog with a list of apps that respond to the intent passed to the createChooser() method and uses the supplied text as the
dialog title.

Receiving an implicit intent

To advertise which implicit intents your app can receive, declare one or more intent filters for
each of your app components with an <intent-filter>
element in your manifest file.
Each intent filter specifies the type of intents it accepts based on the intent's action,
data, and category. The system delivers an implicit intent to your app component only if the
intent can pass through one of your intent filters.

Note: An explicit intent is always delivered to its target,
regardless of any intent filters the component declares.

An app component should declare separate filters for each unique job it can do.
For example, one activity in an image gallery app may have two filters: one filter
to view an image, and another filter to edit an image. When the activity starts,
it inspects the Intent and decides how to behave based on the information
in the Intent (such as to show the editor controls or not).

Each intent filter is defined by an <intent-filter>
element in the app's manifest file, nested in the corresponding app component (such
as an <activity>
element). Inside the <intent-filter>,
you can specify the type of intents to accept using one or more
of these three elements:

You can create a filter that includes more than one instance of
<action>,
<data>, or
<category>.
If you do, you need to be certain that the component can handle any and all
combinations of those filter elements.

When you want to handle multiple kinds of intents, but only in specific combinations of
action, data, and category type, then you need to create multiple intent filters.

An implicit intent is tested against a filter by comparing the intent to each of the
three elements. To be delivered to the component, the intent must pass all three tests.
If it fails to match even one of them, the Android system won't deliver the intent to the
component. However, because a component may have multiple intent filters, an intent that does
not pass through one of a component's filters might make it through on another filter.
More information about how the system resolves intents is provided in the section below
about Intent Resolution.

Caution: Using an intent filter isn't a secure way to prevent other apps from starting
your components. Although intent filters restrict a component to respond to only
certain kinds of implicit intents, another app can potentially start your app component
by using an explicit intent if the developer determines your component names.
If it's important that only your own app is able to start one of your components,
do not declare intent filters in your manifest. Instead, set the
exported attribute
to "false" for that component.

Similarly, to avoid inadvertently running a different app's
Service, always use an explicit intent to start your own service.

Note:
For all activities, you must declare your intent filters in the manifest file.
However, filters for broadcast receivers can be registered dynamically by calling
registerReceiver(). You can then unregister the receiver with unregisterReceiver(). Doing so allows your app
to listen for specific broadcasts during only a specified period of time while your app
is running.

Example filters

To demonstrate some of the intent filter behaviors, here is an example
from the manifest file of a social-sharing app:

The first activity, MainActivity, is the app's main entry point—the activity that
opens when the user initially launches the app with the launcher icon:

The ACTION_MAIN action
indicates this is the main entry point and does not expect any intent data.

The CATEGORY_LAUNCHER category indicates that this activity's
icon should be placed in the system's app launcher. If the <activity> element
does not specify an icon with icon, then the system uses the icon from the <application>
element.

These two must be paired together in order for the activity to appear in the app launcher.

The second activity, ShareActivity, is intended to facilitate sharing text and media
content. Although users might enter this activity by navigating to it from MainActivity,
they can also enter ShareActivity directly from another app that issues an implicit
intent matching one of the two intent filters.

Using a pending intent

A PendingIntent object is a wrapper around an Intent object. The primary purpose of a PendingIntent
is to grant permission to a foreign application
to use the contained Intent as if it were executed from your
app's own process.

Declaring an intent to be executed when the user performs an action with your
App Widget
(the Home screen app executes the Intent).

Declaring an intent to be executed at a specified future time (the Android
system's AlarmManager executes the Intent).

Just as each Intent object is designed to be handled by a specific
type of app component (either an Activity, a Service, or
a BroadcastReceiver), so too must a PendingIntent be
created with the same consideration. When using a pending intent, your app doesn't
execute the intent with a call such as startActivity(). Instead, you must declare the intended component type when you create the
PendingIntent by calling the respective creator method:

To pass this filter, the action specified in the Intent
must match one of the actions listed in the filter.

If the filter does not list any actions, there is nothing for an
intent to match, so all intents fail the test. However, if an Intent
does not specify an action, it passes the test as long as the filter
contains at least one action.

Category test

To specify accepted intent categories, an intent filter can declare zero or more
<category> elements, as shown in the following example:

For an intent to pass the category test, every category in the Intent
must match a category in the filter. The reverse is not necessary—the intent filter may
declare more categories than are specified in the Intent and the
Intent still passes. Therefore, an intent with no categories
always passes this test, regardless of what categories are declared in the filter.

Note:
Android automatically applies the CATEGORY_DEFAULT category
to all implicit intents passed to startActivity() and startActivityForResult().
If you want your activity to receive implicit intents, it must
include a category for "android.intent.category.DEFAULT" in its intent filters, as
shown in the previous <intent-filter> example.

Data test

To specify accepted intent data, an intent filter can declare zero or more
<data> elements, as shown in the following example:

Each <data>
element can specify a URI structure and a data type (MIME media type).
Each part of the URI is a separate
attribute: scheme, host, port,
and path:

<scheme>://<host>:<port>/<path>

The following example shows possible values for these attributes:

content://com.example.project:200/folder/subfolder/etc

In this URI, the scheme is content, the host is com.example.project,
the port is 200, and the path is folder/subfolder/etc.

Each of these attributes is optional in a <data> element,
but there are linear dependencies:

If a scheme is not specified, the host is ignored.

If a host is not specified, the port is ignored.

If both the scheme and host are not specified, the path is ignored.

When the URI in an intent is compared to a URI specification in a filter,
it's compared only to the parts of the URI included in the filter. For example:

If a filter specifies only a scheme, all URIs with that scheme match
the filter.

If a filter specifies a scheme and an authority but no path, all URIs
with the same scheme and authority pass the filter, regardless of their paths.

If a filter specifies a scheme, an authority, and a path, only URIs with the same scheme,
authority, and path pass the filter.

Note: A path specification can
contain a wildcard asterisk (*) to require only a partial match of the path name.

The data test compares both the URI and the MIME type in the intent to a URI
and MIME type specified in the filter. The rules are as follows:

An intent that contains neither a URI nor a MIME type passes the
test only if the filter does not specify any URIs or MIME types.

An intent that contains a URI but no MIME type (neither explicit nor inferable from the
URI) passes the test only if its URI matches the filter's URI format
and the filter likewise does not specify a MIME type.

An intent that contains a MIME type but not a URI passes the test
only if the filter lists the same MIME type and does not specify a URI format.

An intent that contains both a URI and a MIME type (either explicit or inferable from the
URI) passes the MIME type part of the test only if that
type matches a type listed in the filter. It passes the URI part of the test
either if its URI matches a URI in the filter or if it has a content:
or file: URI and the filter does not specify a URI. In other words,
a component is presumed to support content: and file: data if
its filter lists only a MIME type.

Note: If an intent specifies a URI or MIME type, the data test will
fail if there are no <data> elements in the <intent-filter>.

This last rule, rule (d), reflects the expectation
that components are able to get local data from a file or content provider.
Therefore, their filters can list just a data type and don't need to explicitly
name the content: and file: schemes.
The following example shows a typical case in which a <data> element
tells Android that the component can get image data from a content
provider and display it:

Filters that
specify a data type but not a URI are perhaps the most common because most available
data is dispensed by content providers.

Another common configuration is a filter with a scheme and a data type. For
example, a <data>
element like the following tells Android that
the component can retrieve video data from the network in order to perform the action:

Intent matching

Intents are matched against intent filters not only to discover a target
component to activate, but also to discover something about the set of
components on the device. For example, the Home app populates the app launcher
by finding all the activities with intent filters that specify the
ACTION_MAIN action and
CATEGORY_LAUNCHER category.
A match is only successful if the actions and categories in the Intent match
against the filter, as described in the documentation for the IntentFilter
class.

Your application can use intent matching in a manner similar to what the Home app does.
The PackageManager has a set of query...()
methods that return all components that can accept a particular intent and
a similar series of resolve...() methods that determine the best
component to respond to an intent. For example,
queryIntentActivities() returns a list of all activities that can perform
the intent passed as an argument, and queryIntentServices() returns a similar list of services.
Neither method activates the components; they just list the ones that
can respond. There's a similar method,
queryBroadcastReceivers(), for broadcast receivers.

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